The present invention relates in general to MOSFET drivers and, more particularly, to an optically isolated N-channel MOSFET driver.
The use of semiconductors in the domain of industrial controls is well established. MOSFET devices used to drive industrial loads must supply large amounts of current to the load, must be capable of withstanding large overvoltage spikes from the load, and must operate at a large supply voltage. To operate in such a hostile environment, the MOSFET device must be large in geometry. As a result, the gate capacitance of the MOSFET is correspondingly large and provides an obstacle to efficiently switching the output state of the device.
In many applications, microprocessors are used as control circuits to MOSFET drivers in order to enhance the controllability of the load. Microprocessors typically operate at supply voltages of 5 volts or less and are therefore very susceptible to noise and extraneous voltage spikes created in a high voltage control circuit environment.
Optical coupling methods have been used to isolate the input of MOSFET drivers from the microprocessor output in an effort to prevent communication of unwanted disturbances therebetween. Typically, the MOSFET driver input includes a photovoltaic array of photosensitive diodes serially connected and driven by a light emitting diode that is controlled by a microprocessor. While a degree of isolation has been achieved, it is well known that the on-switching time of the MOSFET device becomes highly dependent upon the voltage generating characteristics of the photovoltaic array. To extract a large charging voltage from the photovoltaic array, a large number of series connected photosensitive diodes is required. As a consequence, the effective array capacitance increases and degrades the performance of the on-switching time of the device.
A complete industrial control circuit also must have the capability of switching to an off state in an efficient manner while maintaining good noise immunity characteristics. Typical turn off circuits that have been used in the past incorporate an active device, such as a bipolar transistor whose collector-emitter is connected across the gate-source of the MOSFET device. The base of the bipolar transistor is coupled through a resistor to the gate of the MOSFET. A photosensitive diode is connected across the base-emitter junction of the bipolar transistor. The turn off circuit is activated upon removal of a light emitting source. As one skilled in the art would ascertain, several areas of concern now must be considered. The voltage generated across the photosensitive diode cannot be maintained at near zero volts, a condition that is required to prevent the turn off circuit from interfering with the on state of the MOSFET device. A typical diode voltage will be in the range of 0.5 volts and therefore is not capable of completely disabling the transistor to which it is connected. It is obvious that some leakage current flows through the bipolar collector-emitter, robbing gate charge from the MOSFET. It is also known that with the bipolar transistor circuit approach, an RC time constant variable has been introduced that will inherently delay the complete turnoff of the MOSFET device. Attempts have been made to solve this problem by incorporating a silicon controlled rectifier (SCR) in place of the bipolar transistor. While this speeds the discharge of the gate capacitance, the circuit remains prone to false triggering from noise spikes passed from the MOSFET device load to the MOSFET gate coupled via the gate source capacitance.
Hence, a need exists for an efficient optically isolated N-channel MOSFET driver with rapid switching characteristics that is impervious to noise and voltage spikes.